1. Field of Invention
The present invention relates to ophthalmic microsurgical instruments and, more particularly, to such surgical instrumentation which continuously monitors and controls internal ocular globe fluid pressure during ophthalmic surgical procedures and the like.
2. General Background
A large number of microsurgical procedures inside the eye are performed through "closed systems" which maintain the integrity and internal pressure of the ocular globe while microsurgical instruments are used to penetrate the eye through one or more small incisions (see FIG. 1). Exemplary functions performed by these instruments are:
Fragmentation--the cutting and separation of ocular tissue, such as the lens in cataract surgery or fibrous and membrane-like growths inside the vitreous (e.g., vitrectomy, membranectomy);
Emulsification--the mechanical digestion of tissue (usually the lens) by means of ultrasound in order to facilitate its removal through small incisions;
Irrigation (infusion)--the introduction of a saline solution into the operating field by means of gravity or positive pressure; and
Aspiration (suction)--the removal of fluid and/or entrained tissue fragments by means of vacuum.
The surgeon combines irrigation and aspiration to transport tissue fragments away from the operating field. He or she also uses these functions to maintain intraocular pressure during the surgical procedure. Control of pressure in irrigation and aspiration is extremely important. If the aspiration suction is too strong (due to excessive vacuum) it may damage endothelial cells during anterior chamber surgery or may cause retinal detachment in vitrectomy procedures. Too high an irrigation pressure or excessive variations in the pressure or flow rate of the irrigation fluid may traumatize ocular tissue.
With traditional instrumentation the level of irrigation flow and range of aspiration vacuum are adjusted by a surgical assistant in response to the surgeon's instruction. Available systems afford the surgeon direct control of these variables, usually by means of fingertip or footpedal switches, and provide visual or audio indications of aspiration pressure (vacuum) and irrigation flow. See, for example, U.S. Pat. No. 4,168,707 entitled "Control Apparatus for Microsurgical Instruments."
Generally, such pressure sensors are located in a remote instrument console located a distance of 1-2 meters from the operating site and connected to it through thin, flexible plastic tubing containing a saline solution. Such remote monitoring of pressure has the potential of introducing significant errors in pressure measurements due to the compliance of the tubing and the inertia and viscosity of the fluid column interposed between the surgical site and pressure sensor location. Such errors become more pronounced when air bubbles and tissue fragments enter the flexible conduit which transmits fluid between the operative site and instrument console.
Since a surgeon must depend primarily on visual observation and feel of the surgical site to guide him in controlling the level of suction and irrigation flow rate, knowledge of the accurate pressure or vacuum forces exerted on the tissue at the operating site would enhance greatly the ease and safety of the procedure. Furthermore, accurate control of intraocular pressure both during intraocular surgery and at the time of final wound closure would help minimize postoperative overpressure and associated dangers to the patient.
While there are many devices which are associated with ophthalmic surgical procedures, none is known which accurately monitors internal ocular pressure during surgery. For example, Russian patent 733,670 teaches the use of a strain gauge in the cutting tip of an ophthalmic surgical instrument and a variable audible signal is generated in response to tissue pressure encountered by the instrument when cutting, but internal pressure is not measured.
U.S. Pat. No. 3,945,375 is directed to an ophthalmic surgical instrument for removing tissue and includes a rotating fluted cutter housed in a probe adapted to be inserted into a portion of the body from which tissue is to be removed. The instrument can supply irrigation fluid through the probe to the area being operated upon and evacuate the material through the probe after being engaged by the cutter, but does not monitor internal pressure.
U.S. Pat. No. 4,117,843 teaches a system which controls the infusion of fluid to a closed operating field such as an eye at a selected predetermined pressure in addition to being able to sever material in the field and for evacuating the severed material in a suspension or emulsion of the infusion fluid. However, internal pressure is not measured.
U.S. Pat. No. 4,168,707 relates to an electronic control for microsurgical instruments which is adapted for use in intraocular surgery. Commands received from a surgeon's foot control unit control the various aspiration functions normally performed manually by a surgical assistant. A typical control system used to perform the infusion and aspiration functions required during intraocular survey is described in detail.
There are also various patents which deal with strain gauges that are used to measure blood pressure. See, for example, U.S. Pat. Nos. 2,959,056; 3,550,583; 3,946,724; and 4,274,423. Blood pressure transducers implantable in arteries or veins are described in U.S. Pat. Nos. 3,724,274 and 3,748,623. U.S. Pat. No. 4,274,423 teaches a catheter for use in determining pressures within blood vessels and the heart. And U.S. Pat. No. 4,175,566 is directed to a fluid velocity flow probe.
U.S. Pat. No. 3,776,238 relates to an instrument with two tubes that are mounted co-axially within one another with an opening adjacent the end of the outer tube. Cutting of the vitreous and fibrous bands in the eye caused by hemorrhaging is performed by a chopping action of the sharp end of the inner tube against the inner surface of the end of the outer tube and the bands are removed by suction through the inner tube. The removed vitreous is continuously replaced by a saline solution introduced into the eye through the instrument.
None of these prior art devices provides an ophthalmic microsurgical instrument which can safely and accurately monitor or control internal fluid pressure during ophthalmic surgery.